Human telomerase, the ribonucleoprotein enzyme that maintains telomeres at chromosome termini, is absent in most normal somatic cells and is present in nearly all cancer cells. Activation of telomerase induces cellular immortalization and is critical for the development and progression of tumors. Thus, inhibition of telomerase activity offers a promising approach for treating nearly all cancers. A better understanding of telomerase is needed to allow rational design of effective inhibitors. In humans, telomerase includes two subunits essential for function: an RNA subunit (human telomerase RNA) and a protein subunit (human telomerase reverse transcriptase). The location and mechanism of assembly of the enzyme complex in cancer cells (which may indicate where and how to target telomerase) is not known. Furthermore, identification of additional essential components of telomerase would provide additional potential targets for inhibition. The major objectives of this proposal are to obtain a detailed understanding of the trafficking and assembly of telomerase in cells, and to develop a class of RNA-based telomerase inhibitors effective at limiting or preventing the growth of cancer cells. In vivo analysis will be performed both in Xenopus oocytes (due to the many technical advantages of the system) and cultured human cells (including primary and cancer cell lines). To address our objectives we have defined the following three specific aims: Aim 1: To investigate the pathway of biogenesis of functional telomerase in vivo Aim 2: To examine the localization and trafficking of key human telomerase components in normal and cancer cells Aim 3: To develop efficacious anti-telomerase ribozymes capable of preventing growth of cancer cells.